Micro-electro-mechanical systems (MEMS) devices and methods are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Specifically, a MEM switch can be fabricated utilizing MEMS technology. MEM switches known in the prior art are of two types, namely, the series and shunt types. The series type 10, FIG. 1, consists of a beam 16 cantilevered from a switch base, or substrate 24. The beam 16 has an electrode 14 disposed on it, acts as one plate of a parallel-plate capacitor and contains under its tip a contact 20. A voltage, known as an actuation voltage, is applied between the beam 16 and an electrode 22 on the switch base 24. In the switch-closing phase, or ON-state, the actuation voltage exerts an electrostatic force of attraction on the beam 16 large enough to overcome the stiffness of the beam. As a result of the electrostatic force of attraction, the beam 16 deflects and the contact under its tip 20 makes a connection that bridges the gap in a transmission line 18 running under it, closing the switch. Ideally, when the actuation voltage is removed, the beam 16 will return to its natural state, breaking its connection with the signal line 18 and opening the switch.
The shunt type MEM switch 30, FIG. 2, consists of a doubly-anchored beam (bridge) or membrane 32 anchored on a substrate 42 and disposed across a set of ground-signal-ground (GSG) traces 40, 38, 34, respectively, known as a coplanar waveguide (CPW) transmission line. In its normal state, the “pass” or ON-state, the bridge 32 is undeflected and the amplitude of the signal propagating down the CPW line and entering at its input 44, is minimally attenuated by capacitive coupling to the bridge 32 and, through it, to ground 40, 34, after passing exiting at its output 46. An actuation voltage applied between the bridge 32 and an insulation-protected electrode 36 disposed on the CPW's signal conductor underneath it 38, exerts an electrostatic force of attraction on the bridge 32 large enough to overcome the stiffness of the beam. As a result the bridge deflects and substantially increases the capacitive coupling of the signal to the bridge 32 and ground 40, 34. The amplitude of the signal propagating down the signal line 38, which enters at the input 44, after it passes the deflected bridge 32 and exits at the output 46, is now maximally attenuated and the switch may be said to be in its “blocking” or OFF-state. Ideally, when the actuation voltage is removed, the beam 32 will return to its natural state, breaking its connection with the signal line 38.
One problem with these switches is that the deflected-to-undeflected phase, or OFF-state in the series type, and ON-state in the shunt type, is not directly controlled, however, and relies on the forces of nature embodied in the spring constant of the beam to bring the beam to the undeflected state. However, the forces of nature are not always predictable and therefore unreliable.
For instance, in some cases once the actuation voltage is removed, stiction forces, (forces of attraction that cause the beam to stick to the contact electrode), between the beam and the contact electrode overcome the spring restoring forces of the beam. This results in the beam sticking to the contact electrode and keeping the beam down when, in fact, it should be undeflected. Prior art cantilever/bridge type switches have no mechanism to overcome stiction forces upon deflecting down.
Another problem associated with prior art switches is a problem intrinsic to the beam's change of state from undeflected to deflected. The operation of the beam is inherently unstable. When deflecting, the beam deforms gradually and predictably, up to a certain point, as a function of the actuation voltage being applied to the switch. Beyond that point, control is lost and the beam's operation becomes unstable causing the beam to pull-in, i.e., to come crashing down onto the secondary electrode. This causes the beam to stick as described above, or causes premature deterioration of the contact electrode. Both conditions impair the useful life of the switch and result in premature failure.
There is a need for a MEM switch that overcomes the problems associated with prior art cantilevered- and bridge-type switches.